Photosensitive elastomer polymer compositions for flexographic printing plates

ABSTRACT

Flexographic photosensitive printing plates are made with formulations comprising triblock polymers of poly(vinylpyridine)-poly(butadiene) or poly(isoprene)-poly(vinylpyridine). The polymers may be quaternized or neutralized with an organic acid. When made with quaternized polymers, the plates after imaging with UV may be developed in aqueous solutions. The acid neutralized polymers offer further advantages as to ease of synthesis, safety and improved resolution.

This application is a continuation-in-part of our copending applicationU.S. Ser. No. 729,976 filed Jul. 15, 1991 for "Photosensitive ElastomerPolymer Compositions for Flexographic Printing Plates", incorporatedherein by reference in its entirety now U.S. Pat. No. 5,223,375.

FIELD OF THE INVENTION

This invention relates to radiation curable polymer formulations, aswell as photosensitive articles having solid surfaces or layers preparedfrom such formulations. The invention also relates to a new class ofcompounds useful in making such formulations.

BACKGROUND OF THE INVENTION

Triblock copolymers having poly(4-vinylpyridine) end blocks and apolybutadiene center block can be prepared by sequential anionicpolymerization of butadiene and of 4-vinylpyridine using anorganolithium initiator; see M. S. Jacovic, "Ionomer-like materialsbased on 4-vinylpyridine copolymers," Maromol. Chem., Rapid Commun, vol.10, 217-225 (1989).

Quaternization of triblock polymers ofpolystyrene-polybutadiene-polyvinylpyridine is disclosed by Ohnuma etal. in Chemical Abstracts, 111(16):134883 (1989).

Quaternization of poly(4-vinylpyridine) is known: Boucher et al.,"Kinetics and Mechanism of the Quaternization of Poly(4-vinylpyridine)with Ethyl, n-Propyl, n-Butyl, n-Hexyl and Benzyl Bromide in Sulfolane",J. Chem. Soc. Faraday Trans. I, 73, pp. 1629-1635 (1977).

Diblock and triblock copolymers of butadiene and 2-isopropenylpyridinehave been synthesized by anionic polymerization, using a difunctionallithium initiator: Soum et al., "Synthesis and Characterization of NewBlock Copolymers of Butadiene and 2-isopropenylpyridine," Polymer, vol.29, pp. 1528-1534 (August, 1988).

Triblock copolymers of polystyrene-polybutadiene (orpolyisoprene)-polystyrene have been used in the preparation ofphotosensitive printing plates: U.S. Pat. No. 4,323,637, Chen et al.,"Use of Cover Sheet and Interposed Flexible Film with Block CopolymerCompositions" (1982). The copolymers were not quaternized, nor were theresulting formulations water-developable.

As noted (e.g., U.S. Pat. No. 4,045,231 et al.) triblock polymers of thetype polystyrene-polybutadiene-polystyrene have been used in printingplates, but have not been quaternized. Such plates are notwater-developable. We have discovered that plates made with polymers ofthe type polyvinylpyridine-polybutadiene orpolyisoprene-polyvinylpyridine are water-developable. So far as can bedetermined, quaternized triblock polymers of the type XYX (i.e., acentral block polymer terminated on each end by two identical polymerblocks) were unknown prior to this invention.

U.S. Pat. No. 4,162,919, Richter et al. (1979), discloses a 2-blockcopolymer of styrene-isoprene (or butadiene) for use in flexographicprinting plates.

U.S. Pat. No. 4,245,027, Takeda et al. (1981), discloses aphotosensitive composition comprising copolymers of vinylpyridine and apolyvinylidene halide, optionally with an olefin, e.g., butadiene orisoprene, plus a light sensitizing element, e.g., iodoform, quinones, ordiazonium salts, capable of forming a quaternary salt with thepolyvinylpyridine on heating or on exposure to actinic light. Theinstant invention differs, inter alia, in that our triblock copolymerhas no polyvinylidene component, and further in that when we quaternizeit is prior to formulation and exposure.

U.S. Pat. Nos. 4,271,259 and 4,272,610, both to Breslow et al. (1981),disclose a copolymer of butadiene or isoprene with vinyl pyridine as acomponent in a photosensitive composition. The copolymer is notspecified as ABA, and it is not quaternized. The composition requires anoxidizable component containing allylic hydrogen and a photooxygenationsensitizer.

U.S. Pat. No. 4,430,417, Heinz et al. (1984), discloses ABA triblockcopolymers in making flexographic printing plates. A isstyrene-butadiene, B is isoprene.

U.S. Pat. No. 4,179,531, Hein (1979), discloses ABA triblock copolymers("Kraton"®) - Shell Chemical Co.) in making printing plates where A is astyrene and B is a diene such as isoprene or butadiene. A polythiol isrequired in the formulation.

U.S. Pat. No. 4,045,231, Toda et al. (1977), discloses astyrene-butadiene-styrene triblock copolymer in a printing plateformulation.

Chem. Abstracts 111(6):39978k, Moeller et al., discloses iodine doped ABdiblock copolymers of poly(2-vinylpyridine) and either polybutadiene orpolystyrene.

Chem. Abstracts 109(10):74182b, Shimazaki et al., discloses triblockcopolymer, polystyrene-polyisoprene-poly(vinylpyridine) quaternized withchloromethylstyrene. Use in photosensitive compositions is notdisclosed.

Chem. Abstracts 104(20):169705k, Y. Mihaki, disclosesisoprene-styrene-4-vinylbenzyldimethylamine block copolymers treatedwith di-iodopropane to prepare an amphoteric ion exchanger.

Chem. Abstracts 103(20):166031f, Lelah et al., discloses quaternizationof triblock poly(styrene-butadiene-4-vinylpyridine), with crosslinkingof the butadiene segment and sulfonation of the styrene block.

Chem. Abstracts 101(6):39114u, Kudose et al., discloses quaternizingtriblock butadiene-styrene-4-vinylpyridine copolymer with methylbromide.

Chem. Abstracts 88(2):75242, Fielding-Russell et al., discloses HClquaternization of butadiene-styrene-2-vinylpyridine block copolymer.

SUMMARY OF THE INVENTION

It is an object of the invention to prepare flexographic printing platesusing formulations comprising triblock polymers of the type ABA or A'BA'where A is an unquaternized poly(vinylpyridine) consisting of repeatingsegments of the structure ##STR1##

It is also an object of the invention to prepare a new class ofcompounds, viz., quaternized triblock polymers of the structure A'BA',where:

A' is a quaternized poly(vinylpyridine) consisting of repeating segmentsof the structure ##STR2##

B is a poly(alkadiene) consisting of repeating segments of the structure##STR3## X is a leaving group; R, R¹, and R³ are independently H or 1-8carbonalkyl;

R² is H, 1-8 carbon alkyl, or aralkyl;

a has a value in the range of about 50-1,000;

b has a value in the range of about 500-10,000.

In the alternative, A' may be reacted with an alkyl sulfonic acid,substituted acetic acid, substituted benzoic acid, or substitutedbenzene or aryl sulfonic acid. The preferred acids are trifluoroaceticacid, p-toluene sulfonic acid and 0-nitrobenzoic acid.

It is a further object of the invention to prepare a photosensitiveflexographic printing plate using the aforesaid ABA and A'BA' polymers.

It is also an object to prepare a water-developable printing plate.

Another object is to prepare a thermoplastic elastomeric block polymerdesigned with a hydrophilic terminal block and which has good physicalproperties and water dispersibility.

Another object is to provide a more efficient reaction to make athermoplastic elastomeric block polymer, and also one that uses lesstoxic chemicals.

Yet another object of this invention is to provide a clear, colorlessthermoplastic elastomeric block polymer useful for making flexographicprinting plates.

A further object is to provide a triblock polymer or prepolymer with aninternal block which is a polymer of high enough molecular weight toprovide elastomeric behavior, and where the terminal blocks providewater washability to the resultant triblock polymer.

The term "leaving group" is used in the conventional sense, e.g., asexplained in Morrison and Boyd, Organic Chemistry, Third Ed., publishedby Allyn and Bacon, Inc., p. 456 (1973). As there described, alkylhalides typically react readily in nucleophilic substitutions, e.g.,"IR:X+:Z⁻ →R:Z+:X⁻ where :Z is a nucleophilic reagent (such as vinylpyridine), R:Z is (in such case) quaternized vinylpyridine, and X is theleaving group. Although in the instant invention X is typically halide(F, Cl, Br, I), R² X can be any compound containing a leaving group,e.g. R² -methane sulfonate; R² -trifluoroacetate; R² -trifluoromethanesulfonate; and the like.

Molecular weights are number average and are determined by comparisonwith a polyisoprene standard of known M_(n) by the gel permeationchromatography technique.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to certain triblock polymers and their use inmaking photosensitive printing plates. In a preferred embodiment, theinvention includes a novel class of triblock polymers and their use inmaking printing plates developable in water.

The novel compounds are quaternizedpolyvinylpyridine-polyalkadiene-polyvinylpyridine triblock copolymers ofthe structure A'BA', where A' and B are as above defined.

Both the ABA and A'BA' compounds can be formulated in a solvent withphotosensitive unsaturated compounds (e.g., reactive monomers such asacrylates and photoinitiators), and the solution can be solvent cast onflexographic plate backings. Alternatively, solvent-free formulationsusing ABA or A'BA' polymers may be extruded as photo-sensitive films.

Reagents required in carrying out the invention are described below.

The ABA Triblock Polymers

The starting ABA triblock polymers that are quaternized by the inventionprocess are in general well known, and can be prepared by knownprocesses. Sequential addition polymerization using a lithium initiatoris considered especially useful, as described by Jacovic and by Soum etal., op. cit. These triblock polymers have the structure ABA, where Aand B are as above defined. Additional details for their preparation aregiven below.

In preferred embodiments A is a segment from poly(4-vinylpyridine) orfrom poly(2-vinylpyridine) and B a segment from polyisoprene orpolybutadiene; a is about 70-475 and for isoprene b is about 1000-5000and for butadiene b is about 1200-6000.

Vinylpyridine is available in various position isomers and with variedsubstituents. Substantially all are useful in preparing the inventionpolymers.

The Quaternizing Compound R² X

A variety of quaternizing materials is available for quaternizing thepyridine nitrogen, including the 1-8 carbon alkyl halides, e.g., methyliodide, methyl bromide, ethyl chloride and the like; and aralkylhalides, e.g., benzyl bromide, benzyl chloride, and the like. Alkyl oraralkyl sulfonates are useful. Substantially any conventionalquaternizing compound is considered workable in the invention.Alkylating agents are generally effective. Broadly speaking, compoundsR² X where X is a leaving group are operable. See "Summary of theInvention", above.

The Quaternization Process

Quaternization proceeds conventionally, using solvents, temperatures,reagent proportions, and reaction times similar to those used in typicalquaternizations of the prior art.

Preferred conditions for the quaternization of reaction of the inventioninclude:

Solvent: chloroform, toluene, methylene chloride, methanol, benzene,tetrahydrofuran, and ethylene chloride. The solvent is not critical.Substantially any solvent that dissolves the initial ABA polymer anddissolves or disperses the quaternized product A'BA' is suitable.

Proportions: Stoichiometric excess of quaternizing compound overtriblock polymer.

Quaternization reaction time: about 1-24 hours.

Quaternization reaction temperature: about 0°-60° C.

Following quaternization, the solvent may be stripped under reducedpressure to yield solvent-free product, or, if desired, the solution maybe used directly in preparing the printing plate formulation.

In the alternative, A' may be reacted with an organic acid. Thisreaction, or "neutralization", is different from quaternization in thesense that the nitrogen on the pyridine ring assumes a positive chargeby bond formation with a proton rather than an alkyl group. The endresult is the same as for quaternization, i.e., formation of ahydrophilic cationomer.

There are several advantages to using the organic acid neutralizationroute as opposed to quaternization.

The reaction with selected organic acids is comparatively quick andefficient. The acid reaction is complete in about 1 hour at roomtemperature. Reaction times for the vinyl pyridine copolymers with alkylhalides can be 24 hours or more or may require heating (about 40° C.) inorder to obtain complete quaternization of the pyridine nitrogen.

The acid reacted polymer system is colorless. Quaternization with methyliodide results in colored systems, presumably due to iodide generation.

The acid reaction synthesis conditions are more convenient.Quaternization with methyl/bromide has to be carried out in a closedpressure system due to the low boiling point of methyl bromide.

The acid reaction synthesis process is safer and more ecologicallysound. All the alkyl halides are toxic chemicals which must be handledwith care.

The polymer systems resulting from the acid reaction are clear. Filmsmade with the quaternized polymers tend to be hazy or opaque. This canlead to incomplete cure in films and a reduced capacity for detail inthe ultimate printing plate.

Since vinyl pyridines are weak bases (pka 3.5 to 4.0) it is preferableto employ organic acids which are fairly strong, i.e., pka<3.0. Thefollowing organic acids could be advantageously employed:

1. Alkyl sulfonic acids, such as methyl sulfonic acid, tri-fluoromethanesulfonic acid, 2,2,2 trifluoro ethane sulfonic acid.

2. Substituted acetic acids, such as mono- and di- or tri-fluoroaceticacid.

3. Benzoic acid and mono- and di-substituted benzoic acids particularlythose having electron donating groups, including but not limited tofluoromethyl- and trifluoromethyl- as well as nitro-benzoic acids.Ortho-substituted forms are preferred over meta and para forms.

The preferred acids are trifluoroacetic acid ("TFA") p-toluenesulfonicacid ("PTSA") and O-nitrobenzoic acid (NBA).

Formulations

For use in printing plates, the invention employs two types offormulations. One type is for solvent casting and uses a solvent. Theother is for extrusion and is solvent-free. For solvent casting, thetriblock polymer, ABA or A'BA', is dissolved in a solvent for furtherprocessing. In the case of A'BA', it can be left in the same solvent inwhich it was quaternized. To this solution is added a photosensitiveunsaturated compound (typically an acrylate), and a photoinitiator. Thissolution can be solvent cast to produce a photopolymer plate, which canthen be imaged and developed to produce solid printing plates.Development can be by water and/or solvent.

Thus, a formulation for solvent casting will include:

(1) Triblock polymer, ABA or A'BA' ;

(2) A solvent, including those solvents useful in the quaternizingprocess, e.g., methylene chloride, chloroform, toluene, methanol,benzene, tetrahydrofuran, ethylene chloride, etc.;

(3) A photosensitive unsaturated compound, as described below; and

(4) A photoinitiator, as described below.

The Photosensitive Unsaturated Compound

This material is a reactive monomer, typically an acrylate. Usefulacrylates include those of the formula:

    (CH.sub.2 ═C(R.sup.4)C(:0)--0--).sub.n -R.sup.1

where R⁴ is H or methyl and R⁵ is an organic moiety having a valence ofn, and n is 1 or more.

Such reactive monomers include, but are not limited to,trimethylolpropane triacrylate, hexanediol diacrylate, 1,3-butyleneglycol diacrylate, diethylene glycol diacrylate, 1,6-hexanedioldiacrylate, neopentyl glycol diacrylate, polyethylene glycol-200diacrylate, tetraethylene glycol diacrylate, triethylene glycoldiacrylate, pentaerythritol tetraacrylate, tripropylene glycoldiacrylate, ethoxylated bisphenol-A diacrylate, trimethylolpropanetriacrylate, dimethylolpropane tetraacrylate, triacrylate oftris(hydroxyethyl) isocyanurate, dipentaerythritol hydroxypentaacrylate,pentaerythritol triacrylate, ethoxylated trimethylolpropane triacrylate,triethylene glycol dimethacrylate, ethylene glycol dimethacrylate,tetraethylene glycol di-methacrylate, polyethylene glycol-200dimethacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycoldimethacrylate, polyethylene glycol-600 dimethacrylate, 1,3-butyleneglycol dimethacrylate, ethoxylated bisphenol-A dimethacrylate,trimethylolpropane trimethacrylate, diethylene glycol dimethacrylate,1,4-butanediol diacrylate, diethylene glycol dimethacrylate,pentaerythritol tetramethacrylate, glycerin dimethacrylate,trimethylolpropane dimethacrylate, pentaerythritol trimethacrylate,pentaerythritol dimethacrylate, pentaerythritol diacrylate,urethane-methacrylate or acrylate oligomers and the like which can beadded to the photopolymerizable composition to modify the cured product.Monoacrylates such as cyclohexyl acrylate, isobornyl acrylate, laurylacrylate and tetrahydrofurfuryl acrylate and the correspondingmethacrylates are also operable as reactive diluents.

Photoinitiators

The formulations comprising the novel materials of this inventionrequire a photoinitiator. A large number are available and useful.

Photoinitiators for the photocurable composition include the benzoinalkyl ethers, such as benzoin methyl ether, benzoin ethyl ether, benzoinisopropyl ether and benzoin isobutyl ether. Another class ofphotoinitiators are the dialkoxyacetophenones . exemplified by2,2-dimethoxy-2-phenylacetophenone, i.e., Irgacure®651 (Ciba-Geigy); and2,2-diethoxy-2-phenylacetophenone. Still another class ofphotoinitiators are the aldehyde and ketone carbonyl compounds having atleast one aromatic nucleus attached directly to the carboxyl group.These photoinitiators include, but are not limited to, benzophenone,acetophenone, o-methoxybenzophenone, acenaphthenequinone, methyl ethylketone, valerophenone, hexanophenone, alpha-phenyl-butyrophenone,p-morpholinopropiophenone, dibenzosuberone,4-morpholinobenzophenone,4'-morpholinodeoxybenzoin, p-diacetylbenzene,4-aminobenzophenone, 4'-methoxyacetophenone, benzaldehyde,alpha-tetralone, 9-acetylphenanthrene, 2-acetylphenanthrene,10-thioxanthenone, 3-acetylphenanthrene, 3-acetylindone, 9-fluorenone,1-indanone, 1,3,5-triacetylbenzene, thioxanthen-9-one, xanthene-9-one,7-H-benz[de]-anthracene-7-one, 1-naphthaldehyde,4,4'-bis(dimethylamino)-benzophenone, fluorene-9-one, 1'-acetonaphthone,2'-acetonaphthone, 2,3-butanedione, acetonaphthene, benz[a]anthracene7.12 dione, etc. Phosphines such as triphenylphosphine andtri-o-tolylphosphine are also operable herein as photoinitiators.

Other Additives

Other additives to the photocurable composition can be included. Toinhibit premature crosslinking during storage of the prepolymercontaining compositions of this invention, thermal polymerizationinhibitors and stabilizers are added. Such stabilizers are well known inthe art, and include, but are not limited to, hydroquinone monobenzylether, methyl hydroquinone, amyl quinone, amyloxyhydroquinone,n-butylphenol, phenol, hydroquinone monopropyl ether, phenothiazine andnitrobenzene, and mixtures thereof. Such additives are used in an amountwithin the range of from about 0.01 to about 2% by weight of theprepolymer. These stabilizers are effective in preventing crosslinkingof the prepolymer composition during preparation, processing andstorage.

The compositions also may contain up to about 50% by weight of an inertparticulate filler which is essentially transparent to actinic light.Such fillers include the organophilic silicas, bentonites, silica andpowdered glass. Such fillers can impart desirable properties to thephotocurable compositions and reliefs on printing plates containingthose compositions.

Summarizing, formulations using the herein described polymers suitablyinclude the following (in parts by weight):

(1) Triblock polymer, optionally quaternized, about 50-99, preferablyabout 75-95;

(2) Photosensitive unsaturated compound, about 0.1-50, preferably about0.1-20;

(3) Photoinitiator, about 0.01-5.0, preferably about 0.1-3.0.

Additionally the formulation contains enough solvent to dissolve theabove materials.

For extrusion, the solvent in omitted.

Preparation of Plate

The photocurable compositions comprising polymer ABA or A'BA' can beshaped and formed as a solid layer of suitable thickness according toconventional solvent casting, i.e., dissolving the composition in asolvent, shaping the solution into a film or plate, and removing thesolvent, e.g., by oven-drying. Alternately, conventional extrusioncalendaring or hot press techniques can be used. Solid layers of thephotosensitive composition in the form of a film can be adhered tosupports such as those comprising polyester, nylon, or polycarbonate.Other suitable supports include woven fabrics and mats, e.g., glassfiber fabrics or laminated materials made of, for example, glass fibersand plastics; and steel or aluminum coated plates. It is preferred thatthe supports be dimensionally stable and resistant to the washoutsolutions.

It may be desirable to protect the photosensitive surfaces fromcontamination by dirt and dust during storage before being exposed andwashed. Such protection may be accomplished by application of a flexibleprotective cover sheet to the side of the photocurable compositionopposite that of the support. In addition, the photocurable compositionscan sometimes be tacky, and in such case it may be desirable to apply arelease film to the surface of the photosensitive layer beforeapplication of the coversheet. The release film may consist of a thin,flexible, water-dispersible polymeric film placed in intimate contact onthe surface of the photocurable composition. Such release films are wellknown in the art.

Exposure and Development

Photosensitive articles comprising a support having a solid layer orsurface comprising the invention photocurable compositions, e.g., solidflexographic printing plates, can be processed by well known techniquesfor imagewise exposure to actinic light. Preferably, the light shouldhave a wavelength of about 230-450 microns. Exposure is through anegative placed between the light source and the photosensitive surface.Suitable sources of light include Type RS sunlamps, carbon arc lamps,xenon arc lamps, mercury vapor lamps, tungsten halide lamps, and thelike.

Exposure periods depend upon the intensity of the actinic light,thickness of the plate, and the depth of the relief desired on theprinting plate. Exposure periods of from 2 to 20 minutes are generallysuitable.

After exposure and removal of the negative, the unexposed areas of thephotosensitive surface can be developed in washout solutions as hereindescribed. As noted, a preferred washout solution for plates containingA'BA' polymer is a dilute aqueous surfactant solution. Plain water(preferably warm) is also useful (preferably with brushing). Thisfeature is particularly advantageous in that it avoids problems ofdisposing of washout solutions containing organic solvents. Washoutsolvents recommended for developing ABA-type plates (i.e., usingunquaternized copolymers) include toluene and tetrachloroethylene.

The following Examples illustrate without limiting the invention.

Examples 1-8 Preparation of ABA Block Copolymers

As noted above, materials and methods for the preparation of the ABAcopolymers used in this invention are known. However, to facilitate anunderstanding of our techniques, we offer below the methods that we usedto prepare the starting ABA triblock copolymers.

The poly(2-vinylpyridine)/polyisoprene andpoly(4-vinylpyridine)/polyisoprene ABA block copolymers were prepared bythe sequential addition of isoprene and vinylpyridine to a difunctionalorganolithium initiator in benzene. The benzene solvent was distilledprior to use from the sodium ketyl of benzophenone. Monomers weredistilled from calcium hydride prior to use. The polymerizations werecarried out in oven-dried round-bottom flasks under dry argonatmosphere.

The difunctional initiator was prepared by dissolvingbis[4-(1-phenylvinyl)phenyl]ether or 1,3-diisopropenylbenzene in 500 mlof dry benzene and adding 2 equivalents of sec-butyllithium.

The isoprene midblock was prepared by the addition of isoprene to thedifunctional organolithium initiator. The end blocks were prepared bythe addition of 2- or 4-vinylpyridine to the living polyisoprenyllithium dianion. The living polymer was terminated by the addition ofdegassed methanol (2ml) and was precipitated by adding the polymersolution to a large excess of methanol. The reaction conditions used inthe above steps for Examples 1-8 are given in Table 2.

A butadiene or other alkadiene midblock may be prepared in essentiallythe same way.

Referring to Table 1, "a" and "b" may be readily calculated from thedata. Thus, in Example 8, to determine "b", "Mn" of the polyisoprenemidblock was found to be 155,000 (GPC, polyisoprene standards). Dividingthe "Mn" with the molecular weight of isoprene (68.12) gives 2275="b".

To determine "a", we can find the total weight of vinylpyridineincorporated into the block copolymer from the "wt% VP" (19.3%) and theweight of isoprene (30.65 g). This calculation gives 7.33 g.vinylpyridine in the sample which divided by the molecular weight(105.14 g/mole) gives moles of vinylpyridine (0.07). Moles ofvinylpyridine divided by moles of the initiator (7.01×10⁻⁴ moles) gives100 and since there are 2 endblocks we get 100×2=200 which is the valueof "a". The total molecular weight is thus 155,000+2×100×105.14=176,000.These figures are approximate.

                                      TABLE 1 (1)                                 __________________________________________________________________________            Initiator.sup.2                                                                         Isoprene  Vinylpyridine        Isoprene Micro               Vinyl   Reaction Conditions                                                                     Reaction Conditions                                                                     Reaction Conditions                                                                            Wt %.sup.5                                                                        structure.sup.5, wt          Ex.                                                                              Position                                                                           g.  °C./hr                                                                       g.   °C./hr                                                                      g.   °C./hr                                                                      M.sub.n.spsp.3                                                                   MWD VP  1,4 3,4                      __________________________________________________________________________    1  4    0.1904                                                                            50/1  30.65                                                                                45/1.5                                                                           11.70                                                                               25/0.5                                                                            195                                                                              1.77                                                                              19.8                                                                              93.7                                                                              7.0                      2  4    0.1887                                                                            50/1  30.65                                                                              45/3 12.3   5/0.5                                                                            217                                                                              1.47                                                                              28.3                                                                              92.7                                                                              7.3                      3  4    0.2821                                                                              60/1.75                                                                           30.65                                                                              45/3 13.31                                                                              5/1  194                                                                              1.49                                                                              24.3                                                                              91.9                                                                              8.1                      4  2    0.1889                                                                              65/1.75                                                                           27.24                                                                              45/3 13.11                                                                              5/1  343                                                                              1.37                                                                              29.5                                                                              93.0                                                                              7.0                      5  2    0.2812                                                                            65/2  30.65                                                                              45/3  6.96                                                                              5/1  151                                                                              1.62                                                                              13.2                                                                              92.0                                                                              8.0                      6  2    0.1891                                                                              65/2.5                                                                            30.65                                                                              45/2 12.18                                                                              5/1  218                                                                              (4) 20.6                                                                              93.0                                                                              7.0                      7  2    0.2647                                                                            65/2  44.27                                                                              45/2 13.90                                                                              5/1  317                                                                              1.77                                                                              16.4                                                                              93.8                                                                              6.2                      8  2    0.1110                                                                            65/2  30.65                                                                              50/2   9.94                                                                               5/0.5                                                                            176                                                                              1.35                                                                              19.3                                                                              92.0                                                                              8.0                      __________________________________________________________________________     Notes:                                                                        (1) 500 ml dried benzene used as solvent in all Examples.                     (2) Bis[4(1-phenylvinyl)phenyl]ether initiator used in Ex's. 1-7;             1,3diisopropenylbenzene in Ex. 8.                                             (3) From GPC (polyisoprene standards) × 1000.                           (4) Bimodal distribution.                                                     (5) From .sup.1 H NMR analysis.                                               MWD = molecular weight distribution; VP = polyvinylpyridine.             

EXAMPLES 9-13 Quaternization of Polyvinylpyridine/Polyisoprene ABA BlockCopolymers

The quaternization of the polyvinylpyridine/polyisoprene blockcopolymers was performed by dissolving the polymer in chloroform ortoluene/methylethylketone and adding an excess of methyl bromide. Thereaction conditions used for Examples 9-13 are given in Table 3.Quaternization of the pyridine ring was verified by IR spectroscopy.

                  TABLE 2                                                         ______________________________________                                        Quaternization of Polyvinylpyridine/Polyisoprene                              ABA Block Copolymers                                                                Using Polymer                                                                              Temperature Time  IR data                                  Ex.   of Ex. No.   °C.  hrs   cm.sup.-1                                ______________________________________                                         9    2            0-25        14    1645                                     10    4            0-25        14    1630                                     11    5            0-25        14    1630                                     12    6            0-25        14    1630                                     13    8            40          20    1630                                     ______________________________________                                         Notes:                                                                        (1) Solvent used in Examples 9-12 was toluene/methylethylketone, 3/1 by       volume. Solvent in Example 13 was chloroform, and the reaction was            conducted in a sealed glass reactor.                                          (2) For Examples 9-12, the reaction was started at 0° C. and over      the course of the reaction, the temperature rose to room temperature, i.e     25° C.                                                            

Examples 14-19 Formulation of ABA and A'BA' Block Copolymers

The ABA block copolymers or quaternized A'BA' block copolymers as thecase may be were formulated with photoinitiator and (meth)acrylatemonomers by adding Irgacure®651 photoinitiator from Ciba-Geigy,hexanediol dimethacrylate and hexanediol diacrylate to a chloroform ortoluene solution of the polymer. The composition of the resultingsolutions (calculated on a weight percent, solvent-free basis) was 89%block polymer, 1% photoinitiator, 5% hexanediol dimethacrylate, and 5%hexanediol diacrylate. The solutions were then solvent cast onto apolyester film and dried. The dried films (not imaged, not developed)were cured by exposure to UV light and their mechanical propertiesevaluated. The results are listed in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Physical Properties of Cured Films                                                                     Resil-                                               Polymer of                                                                             Tensile                                                                           Elong.                                                                            Mod.                                                                             Shore A                                                                            ience                                                                             Swell Resistance.sup.1                           Ex.                                                                              Ex. No.                                                                             (psi)                                                                             (%) (psi)                                                                            (°)                                                                         (%) H.sub.2 O                                                                        IPA.sup.2                                                                        EtOH/EtAc.sup.3                                                                      Aq. Ink                                                                            Solvent Ink                    __________________________________________________________________________    14  1    370 586 274                                                                              39   50  6  36 59     11   61                             15  9    663 217 578                                                                              62   56  15 5  13     28    8                             16 10    976 331 778                                                                              54   50  25 26 17     40   19                             17 11    362 271 292                                                                              46   54  6  9  25      9   15                             18 12    393 248 316                                                                              47   49  9  8  30     11   16                             __________________________________________________________________________     Notes:                                                                        All polymers were quaternized except in Ex. 14.                               Tensile strength (Tensile), Elongation (Elong.), and Tensile modulus          (Mod.) (1% elongation) were determined by ASTM D 412.                         Shore hardness was determined by ASTM D 2240.                                 .sup.1 Swell Resistance determined by 24hour weight gain of 20 mil thick      sample.                                                                       .sup.2 Isopropyl alcohol.                                                     .sup.3 85 wt % ethanol, 15 wt % ethyl acetate.                           

EXAMPLE 20

The quaternized poly(4-vinylpyridine)/polyisoprene A'BA' block copolymerof Example 9 was formulated as Example 15 and was solvent-cast to give a60 mil thick solid photocurable film. This film was actinically exposedto UV light in an image-wise fashion and washed in an aqueous surfactantsolution (Triton RW-150) at 120° F. to form a flexographic relief plate.The developed plate showed good reproduction of the negative film image.In the washing operation, a surfactant is not necessary, but ispreferred because it speeds the developing process and gives a cleanerplate. Plain water will do.

EXAMPLES 21-27 Neutralization of Polyvinylpyridine/Polyisoprene ABABlock Copolymers and Their Formulations

The block copolymers similar to the ones in Examples 1-8 were dissolvedin a toluene-methanol (95:5 wt %) solvent mixture. A calculated amountof the strong acid was dissolved in methanol and added dropwise to theabove polymer solution. Enough acid was utilized to neutralize 60 mol %of the vinylpyridine end blocks. Upon addition of the acid solutionthere was seen an immediate increase in the viscosity of the polymersolution. Table 4 summarizes the reaction conditions used. The reactionbetween the polymer and acid was allowed to proceed for 1 hour at R.T.and further formulated as before (Examples 14-19).

The physical properties and swell data of the acid neutralized blockcopolymers are listed in Table 5.

                                      TABLE 4                                     __________________________________________________________________________    Reaction Conditions for the Neutralization.sup.1 of                           Polyvinylpyridine/Polyisoprene ABA Block Copolymers                                     Mn  Wt %     Temperature                                                                          Time                                                                             IR Data                                      Example                                                                            Endblock                                                                           × 10.sup.3                                                                  Endblock                                                                           Acid                                                                              °C.                                                                           Hrs                                                                              CM.sup.1                                     __________________________________________________________________________    21   2 VP  81 32   TFA.sup.2                                                                         25     1  1690,1195                                    22   2 VP 180 24   TFA.sup.2                                                                         25     1  1690,1195                                    23   2 VP 126 30   TFA.sup.2                                                                         25     1  1690,1195                                    24   2 VP 126 30   TFA.sup.2                                                                         25     1  1690,1195,1125                               25   4 VP 115 28   TFA.sup.2                                                                         25     1  1690,1195                                    26   4 VP  91 25   TFA.sup.2                                                                         25     1  1690,1195                                    27   4 VP  91 25   TFA.sup.2                                                                         25     1  1690,1195,1125                               __________________________________________________________________________     .sup.1 60% Neutralized                                                        .sup.2 Trifluoracetic Acid                                                    .sup.3 ptoluenesulfonic Acid                                             

                                      TABLE 5                                     __________________________________________________________________________    Physical Properties of 60% Neutralized Block Copolymers                       Polymer of                                                                           Elongation                                                                          Tensile                                                                           Modulus                                                                            Shore                                                                             Resilience                                                                          Swells %                                      Example No.                                                                          %     PSI PSI  A°                                                                         %     Aqueous Ink                                                                          Solvent Ink                            __________________________________________________________________________    21      80   445 320  55  37    35     53                                     22     475   1100                                                                              350  57  51    25     32                                     23     288   456 212  39  41    38     50                                     24     306   1010                                                                              362  64  38    38     59                                     25     215   450 185  49  40    28     15                                     26     245   490 205  44  43    26     18                                     27     200   200 225  52  26    24     19                                     __________________________________________________________________________

The advantages of using neutralization with strong acids overquaternization becomes quite apparent from Tables 4 and 5. The physicalproperties (tensile, modulus, Shore Aπ, etc.) were seen to be within therange and similar to the quaternized block copolymers. The physicalproperties, of course, depend on the total Mn and Wt % VP. Theformulated neutralized block copolymers were non-haze and clear. Thusincomplete through-cure was avoided, and the resolution of the imagedplate was improved. The acids used were either solids or high-boilingliquids and non-toxic. The reaction proceeded under relatively mildconditions (1 hour at room temperature) and did not require a closedpressure system.

What is claimed is:
 1. A photosensitive flexographic printing platecomprising a backing and thereon a layer of a mixture comprising aquaternized triblock copolymer of the structure.
 2. The plate of claim1, wherein the organic acid is trifluoroacetic acid, p-toluenesulfonicacid, o-nitrobenzoic acid or a mixture of the above.